1. Field of the Invention
The present invention relates to a transistor-transistor logic (TTL) circuit and, more particularly, to a TTL circuit in which an output characteristic of an output transistor is improved.
2. Description of the Related Art
A known typical TTL circuit includes an output transistor, a phase-splitting transistor having an emitter connected to the base of the output transistor and a collector connected through a resistor to a higher voltage power supply line, and an input transistor for receiving an input signal and transmitting a change thereof to the base of the phase-splitting transistor. In this arrangement, the potential at the collector of the output transistor serves as an output of the TTL circuit.
For example, assume that when the input signal is at a logic low level (hereinafter referred to as L level), the input transistor is ON and the phase-splitting transistor is OFF. In this state, the output transistor is OFF and the output of the TTL circuit is at a logic high level (hereinafter referred to as H level), and when the input signal changes from the L level to the H level, the input transistor is turned OFF and the phase-splitting transistor is turned ON. As a result, currents are fed from the power supply line through the resistor and phase-splitting transistor to the base of the output transistor, the output transistor is turned ON, and the output goes from the H level to the L level.
Therefore, an external circuit connected to the collector of the output transistor is driven by the ON and OFF operation of the output transistor, which operates according to the amount of base current thereof. In this case, to ensure that the external circuit is driven with a high reliability, a sufficient base current must be fed to the output transistor, to increase the driving ability thereof. When the driving ability is increased simply by increasing the base current, however, an overshoot or ringing often occurs in the output waveform. This may cause a mis-operation in a subsequent circuit to be connected to the collector of the output transistor. Also, a quick change in currents may cause electromagnetic waves resulting in the appearance of noise.